Polymorphisms of the mouse which influence the response in vivo to Friend erythroleukemia virus (FV) is the focus of these studies, but since disease induction represents an interaction between the host and viral genomes, viral polymorphisms is also being studied in relation to different host genotypes. Comparisons will be made among erythroleukemias and cell lines established from them in mice of different genotypes at the Fv-2 and Fv-4 loci, for example, and using strains of spleen focus-forming virus (SFFV) with distinguishable gene products. Since such cell lines generally cease to express all or part of their proviral genomes during serial passage, we will continue our exploration of the mechanisms involved in this shutdown. We will pursue our earlier findings suggesting that virions containing SFFV or helper virus genomes differ in ways that may be useful in separating them from each other. Having established a BALB.D2-Fv-1n congenic strain, comparisons will be made with parental strain BALB/c mice with respect to virological and disease susceptibility status. Studies designed to identify new genes influencing FV erythroleukemogenesis will also be pursued.